Method for forming threads on plastic nipples

ABSTRACT

Apparatus is provided to perform the following work steps in sequence. First, one end of a plastic tube is heated while under circumferential compression. The compression deforms the heated end inwardly and when the apparatus exerting the compressive force is removed, the heated end is inserted in, and gripped by segmented dies having pipe threads on their gripping surfaces. An expansion tool is inserted into the heated end of the tube and an elastomeric annulus which is part of the tool is compressed axially. Said axial compression displaces the annulus radially against the inner surface of the tube which, in turn, is expanded radially and deformed into the thread form of the dies. Next the axial pressure on the annulus is released, the annulus recedes radially and the expansion tool is retracted from the tube end. A cooling probe is inserted from the opposite end to hold the threaded tube in place as the segmented dies are separated. The cooling probe will freeze the heated end of the tube such that the thread form will be permanently fixed in the exterior tube surface.

nited States Patent [191 Edgar Dec. 24, 1974 METHOD FOR FORMING THREADSO PLASTIC NIPPLES 1 James S. Edgar, 1306 S. Columbus Airport No. 3,Columbus, Ohio 43207 22 Filed: Mar. 12,1973

21 Appl. No.: 340,462

[76] Inventor:

Primary Examiner-Richard R. Kucia Attorney, Agent, or FirmFay & Sharpe[57] ABSTRACT Apparatus is provided to perform the following work stepsin sequence. First, one end of a plastic tube is heated while undercircumferential compression. The compression deforms the heated endinwardly and when the apparatus exerting the compressive force isremoved, the heated end is inserted in, and gripped by segmented dieshaving pipe threads on their gripping surfaces. An expansion tool isinserted into the heated end of the tube and an elastomeric annuluswhich is part of the tool is compressed axially. Said axial compressiondisplaces the annulus radially against the inner surface of the tubewhich, in turn, is expanded radially and deformed into the thread formof the dies. Next the axial pressure on the annulus is released, theannulus recedes radially and the expansion tool is retracted from thetube end. A cooling probe is inserted from the opposite end to hold thethreaded tube in placeas the segmented dies are separated. The coolingprobe will freeze the heated end of the tube such that the thread formwill be permanently fixed in the exterior tube surface.

4 Claims, 8 Drawing Figures l l l l l l PATENIEDUEBWW 3.856.906

sum 3 BF 'IIIIIIIIIIIIIIIIIIIIA ll iillllllllllllllm METHOD FOR FORMINGTHREADS ON PLASTIC NIPPLES BACKGROUND OF THE INVENTION Metallic tubesare traditionally used in plumbing but, more recently plastic tubing hasbecome more in demand particularly PVC plastic tubing and other lessexpensive tubing of thermoplastic polymers. The reasons for thedesirability of plastic tubing is its light weight and low corrosionpotential. The detrimental factors include the difficulty ofjoining andseparating the tube lengths.

One often used mechanism for joining is bonding by chemical reactionwhere the end of the tube and an associated sleeve are painted with somereacting chemical. When the tube is inserted into the sleeve, thechemicals react to bond the two together. Unfortunately, getting themapart again requires cutting because the bond is permanent.Additionally, if for some reason the chemical reaction does not work tocompletely seal the joint, leakage occurs and severing is required toreplace the sleeve. There is no way to tighten the frozen joint nor anyeconomically practical way to put an additional seal at the joint.

More recently, threaded plastic tubing has been used in the same way asthreaded metallic tubing and this solves the problem to a great extent.Thus, there is a critical need for threaded nipples of standard lengthssuch as 3 inches, 6 inches, 1 foot, etc., at various standard diameters.Unfortunately, there is substantial difficulty in threading plasticnipples, and that is the reason for the aforementioned need.

The inexpensive thermoplastic polymers economically practical for use inplastic tubing are not nearly as strong in tension and compression asequivalent copper or steel tubing. As a consequence, when nipples are tobe threaded, they require gripping to prevent rotation when thethreading die cuts the end, internal support to prevent either thecollapse of the tube or inadequate depth on the cut threads, or somecompensation for the problem. It goes without saying that this becomes avery difficult problem when one considers trying to thread nipples aquarter inch in diameter and in close or short lengths.

This invention solves the problems by approaching them with an entirelynew and different theory which avoids the gripping and collapse problemsinherent in cutting threads in a conventional manner.

BRIEF DESCRIPTION OF THE INVENTION The apparatus performing the methodof this invention includes a reamer which, initially cuts afrustoconical surface inside the end of the tube to be threaded. Thereason for this will be explained subsequently. Then, a clamping forceis circumferentially applied on the exterior surface of the tube andtogether they are heated to a temperature which will soften the plasticbut, will not be so high as to melt it.

After the tube is heated to an adequate temperature, it is removed fromthe heating means and the clamping force removed. The heated end of thetube is then inserted into and gripped by segmented dies having a pipethread pattern on its interior gripping surface.

With the tube locked in place, a force or expansion tool is insertedinto the heated end of the tube which, when actuated, will force thetube end radially outward to completely fill the thread form of the die.When the force tool is removed and the tube is cooled by a probeprojecting from the opposite end of the tube, the segmented dies areretracted and the threaded nipple adequately cooled and placed instorage or elsewhere for subsequent use or for threading the other endif the need arises.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view ofasingle operator, table model of the apparatus of this invention;

FIG. 2 is a fragmentary sectional view showing a tube end being reamedprior to its heating;

FIG. 3 is a fragmentary sectional view of a heated tube with acircumferential clamp around the heat end;

FIG. 4 is a fragmentary sectional view of the heated end ofa plastictube clamped in threading forming dies;

FIG. 5 is a fragmentary sectional view similar to FIG. 4 but, with anexpanding tool inserted into the heated end of the tube and the tubebeing shown expanded radially into firm contact with the thread formpattern;

FIG. 6 is a fragmentary view of the threaded nipple of FIG. 5 with thedies retracted and the nipple being removed by a cooling probe;

FIG. 7 is a perspective view of the preferred gripping clamp of thisinvention; and

FIG. 8 is a fragmentary sectional view of the force or exxpanding toolof this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring particularly to FIG.1, most of the apparatus to accomplish this invention is illustratedmounted on a table top 10. It includes a heater 12, illustrated withthree nipples l4 projecting therefrom. Split or segmented dies 16 areillustrated as being mounted on a frame 18 and actuated by hydraulicmeans 20. Hydraulic fluid will obviously be delivered from a reservoirunder pressure to means'20; the piping. control mechanism, etc. are notillustrated as any skilled mechanic could design an adequate system andthey are not a part of the invention. As will be clear to those havingordinary skill in the art, the frame 18 and hydraulic means 20 are butone of a plurality of equivalent means for accomplishing the resultdesirable which, will be explained in more detail subsequently.

A force tool 22 is aligned with the dies 16 in guide channel 24 and acooling probe 26 is in a similar guide channel 28 on the opposite sidefrom the force tool.

For convenience, the invention will be described hereinafter by thepreferred process used in making the threaded nipple. First, let it besaid that the invention being described is concerned with forming pipethreads on plastic nipples. American Standard pipe threads diverge fromthe axis of a pipe at an angle of approxiamtely l 47 or an includedangle of 334. With metallic nipples, the excess metal is cut away from acylindrical periphery by thread dies which rotate relative to the tube.As a general rule, such a procedure puts too much pressure on theconventional plastic tube and will often cause collapse or tearing. Thisinvention overcomes that complex problem and one of the first thingsdone in furtherance of this objective is illustrated in FIG. 2; that is,a reaming tool 30 initially reams the inside end surface of the tubewhich is to be threaded. It is preferred that the reaming tool cut afrustoconical surface which will diverge toward the open end of the tubeat an angle not substantially greater than 3 /2. The

reason for this is that the usual amount of material removed from theexterior surface of a metallic tube by cutting dies is to be removed inthis casefrom the interior surface by the reamer. In sequence, this willbe followed by a radial contraction of the end to be threaded andsubsequently, by a radial expansion.

The intention of this combination of three steps, namely, reaming,contracting and expanding is to have a relatively uniform interior tubediameter from end to end. As will be explained subsequently, the radialexpansion of the tube which forms the threads could result in a seriesof undulations in the fluid flow path with every joint in the pipe.Obviously, this is undesirable because, it would create undesirableturbulence within the fluid itself.

As a consequence of this desire for a smooth interior, the reamer isfirst used to cut the stated amount from the inside surface of the tube14 as illustrated in FIG. 2. Next a modified split hose-type clamp 32 iscontracted around the exterior surface of the reamed end with theresulting inward radial pressure and deformation illustrated in FIG. 3.

The clamp itself is more clearly illustrated in FIG. 7. Because it iswell known to those in the automobile repair and plumbing trades nodetailed explanation of the clamp appears necessary, except to say thata spring steel rod 34 combines with the split metallic sleeve 36 toprovide a semi-controlled compression tool in the obvious way.

Referring to FIG. 1, it will be seen that each nipple 14 has one endinserted in the heater 12. The inserted end has a clamp 32 mountedthereon as illustrated in FIG. 3. The clamp 32 serves two purposes. Thecompression feature has been discussed earlier. However, it also servesone other very useful and important purpose. That is, it shields the endof the tube from the radiant heat of the heating element within theheater 12. In addition because the sleeve 36 is metallic, the heat isrelatively uniformly distributed around the periphery of the tube, metalbeing a good heat conductor. As is well known, plastic per se is not agood conductor of heat and hot spots might result without the metallicband at the periphery. Radiant electric heating is not a criticalfeature of the invention but is preferred.

After the appropriate temperature is generated on the tube it can beremoved for subsequent work. The temperature desirable for subsequentwork will vary from one material to another. For example, a temperatureof about 300 F. is satisfactory for polyvinyl chloride and about 320 F.is suitable for polypropylene. Exact parameters can be tailored for theparticular materials and machines in use but, the principals set outremain the same.

After the tube is taken from the heater and the clamp removed, theheated end of the tube is inserted into the split die 16 and thehydraulic means 20 is actuated to grip the hot end. It will of course.be recognized that the FIG. I showing of two split dies is merelyillustrative of the concept. The split dies copuld be of three or fouror five segments or any other number that was found convenient. Inaddition. the illustration of the die working on only one end of thetube in FIG. 4 can clearly be modified such that the dies can operate onboth ends of the tube simultaneously. There is no apparent need toillustrate this series of possible modifications, as the principals arenot changed.

Taking the nipple from the heated environment of the heater l2 andinserting it into dies 16 could cause the nipple 14 to cool and hardentoo soon because, the metallic dies 16 would seve as a heat sink. Forthis reason, heating elements 38 are preferably incoporated into thedies to keep them at a temperature of 275300 F. and thus, they will drawoff very little heat from the tube itself.

One end of the force tool 22 is inserted into the tube 14 as illustratedin FIG. 5. The portion illustrated includes a rod 40 coaxially mountedwithin a hollow rod 42. A deformable resilient, relativelyincompressable annulus 44 circumscribes the rod 40 and is sandwichedbetween two radially extending flanges 46 and 48. The flange 48 ismerely a washer held in place by a nut 50 threaded onto the end of rod40. v

In operation, the tube 14 is clamped into the dies I6 and the force tool24 slides forward on the channel 24 until the elastomeric annulus 44slides into the end of tube 14 and flange 46 abutts the tube end or thedie face. Handle 52, is pushed downward, thereby retracting rod 40relative to hollow rod 42 which, deforms the resilient annulus 44axially. Because the annulus is relatively incompressable, it expandsradially as it contracts axially; this forces it into contact with theinterior surface of the nipple and in turn forces the interior surfaceof the nipple radially outward into the thread pattern 54 where it isplastically deformed into the desirable shape.

After adequate force has been transmitted and the threads properlyformed, the handle 52 is raised allowing the annulus 44 to contractradially and the force tool is retracted from the end of the nipple. Atthat point, the cooling probe 26 is inserted into the tube from itsopposite end. Many means of cooling the probe are within thecontemplation of this invention but, no specific cooling mechanism ortubing arrangement has been illustrated, as such is obvious. In anycase, as illustrated in FIG. 6, when the cooling probe is inserted andit sufficiently cools the tube to freeze the threads 56, the dies 16 maybe retracted and the probe 26 will hold the nipple in place where it maybe removed for storoage or whatever need may arise.

While the inventive concept illustrated and described with greatparticularity in this specification has been limited to forming pipethreads on the end of a plastic tube, it will be recognized by thosehaving ordinary skill in the art, similar processes and modifiedequipment may be used to form flanges, cupped or belled ends, grooves,notches, splines, decorations, etc., on the ends of plastic tubes.Certain of these shapes would require modifications of the annulus 44and certainly these fall well within the skill of the art and are withinthe contemplation of this invention. Additionally, the clamp 32 could beeliminated with adequate safeguards on heat distribution and someapparatus for performing the heated end of the nipple prior to itsinsertion into dies 16.

As will be obvious to those having ordinary skill in the art. certainadditional modifications may be made in the method and apparatusdescribed without departing from the spirit of the invention. It is notthe inventors intention to be bound by the language used in describingthe invention nor the single illustrated embodiment. Rather he intendsto be bound by only the appended claims.

I claim:

1. A process of forming threads on the end of a cylindrical plastic-tubecomprising:

a. removing material from the inside surface of the tube at the end tobe threaded, said removal comprising cutting the material away to form afrusto conical interior surface diverging toward the open end of thetube at an angle not substantially greater than 3 /2,

b. heating the end of the tube to be threaded and deforming it radiallyinward to reform the interior surface of the tube to roughly acylindrical surface while reforming the exterior surface at the end ofthe tube to a frusto conical surface converging toward the end of thetube,

c. gripping the heated and deformed end of the tube with segmented dies,said dies having a thread form pattern on their inner surface in contactwith said heated tube end,

d. applying radial pressure from within the tube against its interiorsurface to plastically deform said tube radially against the dies withforce sufficient to cause plastic flow of the heated end into the threadform pattern of said dies and e. cooling the tube by application ofcooling fluid to quickly freeze the thread form pattern on the end ofthe tube and prevent damage to the thread due to the soft nature of thehot plastic.

2. The process of claim 1 including placing a circumferentiallyextending clamp on the tube end prior to its insertion into said dies tocompress the heated end radially inwardly and removing the clamp priorto placing the tube within said dies 3. The process of claim 2 whereinthe expanding step is accomplished by inserting annulus of resilient,substantially, incompressable material into the heated end of said tubeand deforming the annulus axially, thereby forcing it to expand radiallyagainst the interior surface of said tube.

4. The process of claim 3 wherein the segmented dies are heatedcontinually to maintain their temperature near that of the heated tubeto prevent premature freezing of the heated end and improperdeformation. l l l

1. A process of forming threads on the end of a cylindrical plastic tubecomprising: a. removing material from the inside surface of the tube atthe end to be threaded, said removal comprising cutting the materialaway to form a frusto conical interior surface diverging toward the openend of the tube at an angle not substantially greater than 3 1/2 *, b.heating the end of the tube to be threaded and deforming it radiallyinward to reform the interior surface of the tube to roughly acylindrical surface while reforming the exterior surface at the end ofthe tube to a frusto conical surface converging toward the end of thetube, c. gripping the heated and deformed end of the tube with segmenteddies, said dies having a thread form pattern on their inneR surface incontact with said heated tube end, d. applying radial pressure fromwithin the tube against its interior surface to plastically deform saidtube radially against the dies with force sufficient to cause plasticflow of the heated end into the thread form pattern of said dies and e.cooling the tube by application of cooling fluid to quickly freeze thethread form pattern on the end of the tube and prevent damage to thethread due to the soft nature of the hot plastic.
 2. The process ofclaim 1 including placing a circumferentially extending clamp on thetube end prior to its insertion into said dies to compress the heatedend radially inwardly and removing the clamp prior to placing the tubewithin said dies.
 3. The process of claim 2 wherein the expanding stepis accomplished by inserting annulus of resilient, substantially,incompressable material into the heated end of said tube and deformingthe annulus axially, thereby forcing it to expand radially against theinterior surface of said tube.
 4. The process of claim 3 wherein thesegmented dies are heated continually to maintain their temperature nearthat of the heated tube to prevent premature freezing of the heated endand improper deformation.